A variety of techniques are available for providing visual displays of graphical or video images to a user. For example, cathode ray tube type displays (CRTs), such as televisions and computer monitors are very common. Such devices suffer from several limitations. For example, CRTs are bulky and consume substantial amounts of power, making them undesirable for portable or head-mounted applications.
Flat panel displays, such as liquid crystal displays and field emission displays, may be less bulky and consume less power. However, typical flat panel displays utilize screens that are several inches across. Such screens have limited use in head mounted applications or in applications where the display is intended to occupy only a small portion of a user's field of view.
One approach to overcoming many limitations of conventional displays is a scanned beam display, such as that described in U.S. Pat. No. 5,467,104 of Furness et al., entitled VIRTUAL RETINAL DISPLAY, which is incorporated herein by reference. As shown in FIG. 1, in a scanned beam display 40, a scanning source 42 outputs a scanned beam of light that is coupled to a viewer's eye 44 by a beam combiner 46. In scanned displays, a scanner, such as a scanning mirror or acousto-optic scanner, scans a modulated light beam onto a viewer's retina. An example of such a scanner is described in U.S. Pat. No. 5,557,444 to Melville et al., entitled MINIATURE OPTICAL SCANNER FOR A TWO-AXIS SCANNING SYSTEM, which is incorporated herein by reference. The scanned light enters the eye 44 through the viewer's pupil 48 and is imaged onto the retina 59 by the cornea. In response to the scanned light the viewer perceives an image.
Sometimes such displays are used for partial or augmented view applications. In such applications, a portion of the display is positioned in the user's field of view and presents an image that occupies a region 43 of the user's field of view 45, as shown in FIG. 2A. The user can thus see both a displayed virtual image 47 and background information 49. If the background light is occluded, the viewer perceives only the virtual image 47, as shown in FIG. 2B.
One difficulty with such displays is raster pinch, as will now be explained with reference to FIGS. 3-5. As shown diagrammatically in FIG. 3, the scanning source 42 includes an optical source 50 that emits a beam 52 of modulated light. In this embodiment, the optical source 50 is an optical fiber that is driven by one or more light emitters, such as laser diodes (not shown). The emitted beam 52 strikes a turning mirror 54 and is directed toward a horizontal scanner 56. The horizontal scanner 56 is a mechanically resonant scanner that scans the beam 52 periodically in a sinusoidal fashion. The horizontally scanned beam then travels to a vertical scanner 58 that scans periodically to sweep the horizontally scanned beam vertically. Eye coupling optics 60 then couple the scanned beam 52 to an exit pupil expander 62 that provides an expanded exit pupil for viewing by a viewer's eye 64. One such expander is described in U.S. Pat. No. 5,701,132 of Kollin, et al., entitled VIRTUAL RETINAL DISPLAY WITH EXPANDED EXIT PUPIL, which is incorporated herein by reference. One skilled in the art will recognize that, for differing applications, the exit pupil expander 62 may be omitted or may have a variety of structures, including diffractive or refractive designs. For example, the exit pupil expander 62 may be a planar or curved structure and may create any number or pattern of output beams in a variety of patterns.
Returning to the description of scanning, as the beam scans through each successive location in a plane 66, the beam color and intensity is modulated in a fashion to be described below to form a respective pixel of an image. By properly controlling the color and intensity of the beam for each pixel location, the display 40 can produce the desired image.
The respective waveforms of the vertical and horizontal scanners are shown in FIGS. 4A and B respectively. In the plane 66 (FIG. 3), the beam traces the pattern 68 shown in FIG. 5. As can be seen by comparing the actual scan pattern 68 to a desired raster scan pattern 69, the actual scanned beam 68 is "pinched" at the outer edges of the plane 66. That is, in successive forward and reverse sweeps of the beam, the pixels near the edge of the scan pattern are unevenly spaced. This uneven spacing can cause the pixels to overlap or can leave a gap between adjacent rows of pixels. Moreover, because image information is typically provided as an array of data, where each location in the array corresponds to a respective position in the ideal raster pattern 69, the displaced pixel locations can cause image distortion.